Method and system for increased bandwidth efficiency in multiple input - multiple output channels

ABSTRACT

In one disclosed embodiment, an input bit stream is supplied to a trellis code block. For example, the trellis code block can perform convolutional coding using a rate 6/7 code. The output of the trellis code block is then modulated using, for example, trellis coded quadrature amplitude modulation with 128 signal points or modulation symbols. The sequence of modulation symbols thus generated can be diversity encoded. The diversity encoding can be either a space time encoding, for example, or a space frequency encoding. The sequence of modulation symbols, or the sequence of diversity encoded modulation symbols, is fed to two or more orthogonal Walsh covers. For example, replicas of the modulation symbol sequences can be provided to increase diversity, or demultiplexing the modulation symbol sequences can be used to increase data transmission rate or “throughput”. The outputs of the Walsh covers are fed as separate inputs into a communication channel.

BACKGROUND

[0001] 1. Field of the Invention

[0002] The present invention generally relates to the field of wirelesscommunication systems. More specifically, the invention relates totransmission for wideband code division multiple access communicationsystems using multiple input multiple output channels.

[0003] 2. Related Art

[0004] In wireless communication systems several users share a commoncommunication channel. To avoid conflicts arising from several userstransmitting information over the communication channel at the sametime, some regulation on allocating the available channel capacity tothe users is required. Regulation of user access to the communicationchannel is achieved by various forms of multiple access protocols. Oneform of protocol is known as code division multiple access (CDMA). Inaddition to providing multiple access allocation to a channel of limitedcapacity, a protocol can serve other functions, for example, providingisolation of users from each other, i.e. limiting interference betweenusers, and providing security by making interception and decodingdifficult for a non-intended receiver, also referred to as lowprobability of intercept.

[0005] In CDMA systems each signal is separated from those of otherusers by coding the signal. Each user uniquely encodes its informationsignal into a transmission signal. The intended receiver, knowing thecode sequences of the user, can decode the transmission signal toreceive the information. The encoding of the information signal spreadsits spectrum so that the bandwidth of the encoded transmission signal ismuch greater than the original bandwidth of the information signal. Forthis reason CDMA is also referred to as “spread spectrum” modulation orcoding. The energy of each user's signal is spread across the channelbandwidth so that each user's signal appears as noise to the otherusers. So long as the decoding process can achieve an adequate signal tonoise ratio, i.e. separation of the desired user's signal from the“noise” of the other users' signals, the information in the signal canbe recovered. Other factors which affect information recovery of theuser's signal are different conditions in the environment for eachsubscriber, such as fading due to shadowing and multipath. Briefly,shadowing is interference caused by a physical object interrupting thesignal transmission path between the transmitter and receiver, forexample, a large building. Multipath is a signal distortion which occursas a result of the signal traversing multiple paths of different lengthsand arriving at the receiver at different times. Multipath is alsoreferred to as “time dispersion” of the communication channel. Multipathfading may also vary with time. For example, in a communication unitbeing carried in a moving car, the amount of multipath fading can varyrapidly.

[0006] A number of methods have been implemented to provide effectivecoding and decoding of spread spectrum signals. The methods includeerror detection and correction codes, and convolutional codes. Inwireless communications, especially in voice communications, it isdesirable to provide communication between two users in both directionssimultaneously, referred to as duplexing or full-duplexing. One methodused to provide duplexing with CDMA is frequency division duplexing. Infrequency division duplexing, one frequency band is used forcommunication from a base station to a mobile user, called the forwardchannel, and another frequency band is used for communication from themobile user to the base station, called the reverse channel. A forwardchannel may also be referred to as a downlink channel, and a reversechannel may also be referred to as an uplink channel. Specificimplementation of coding and modulation may differ between forward andreverse channels.

[0007] The information in the user's signal in the form of digital datais coded to protect it from errors. Errors may arise, for example, as aresult of the effects of time-varying multipath fading, as discussedabove. The coding protects the digital data from errors by introducingredundancy into the information signal. Codes used to detect errors arecalled error detection codes, and codes which are capable of detectingand correcting errors are called error correction codes. Two basic typesof error detection and correction codes are block codes andconvolutional codes.

[0008] Convolutional codes operate by mapping a continuous informationsequence of bits from the digital information of the user's signal intoa continuous encoded sequence of bits for transmission. By way ofcontrast, convolutional codes are different from block codes in thatinformation sequences are not first grouped into distinct blocks andencoded. A convolutional code is generated by passing the informationsequence through a shift register. The shift register contains, ingeneral, N stages with k bits in each stage and n function generators.The information sequence is shifted through the N stages k bits at atime, and for each k bits of the information sequence the n functiongenerators produce n bits of the encoded sequence. The rate of the codeis defined as R=k/n, and is equal to the input rate of user informationbeing coded divided by the output rate of coded information beingtransmitted. The number N is called the constraint length of the code;complexity—or computing cost—of the code increases exponentially withthe constraint length. A convolutional code of constraint length 9 andcode rate 3/4, for example, is used in some CDMA systems.

[0009] The highly structured nature of the mapping of the continuousinformation sequence of bits into continuous encoded sequence of bitsenables the use of decoding algorithms for convolutional codes which areconsiderably different from those used for block codes. The codingperformed by a particular convolutional code can be represented invarious ways. For example, the coding may be represented by generatorpolynomials, logic tables, state diagrams, or trellis diagrams. If thecoding is represented by a trellis diagram, for example, the particulartrellis diagram representation will depend on the particularconvolutional code being represented. The trellis diagram representationdepends on the convolutional code in such a way that decoding of theencoded sequence can be performed if the trellis diagram representationis known.

[0010] For signal transmission, convolutional coding may be combinedwith modulation in a technique referred to as “trellis codedmodulation”. Trellis coded modulation integrates the convolutionalcoding with signal modulation in such a way that the increased benefitof coding more than offsets the additional cost of modulating the morecomplex signal. One way to compare different methods of signaltransmission is to compare the bandwidth efficiency. Bandwidthefficiency is typically measured by comparing the amount of informationtransmitted for a given bandwidth, referred to as the “normalized datarate”, to the SNR per bit. The maximum normalized data rate that canpossibly be achieved for a given SNR per bit is the theoretical maximumcapacity of the channel, referred to as the “Shannon capacity” of thechannel. The more bandwidth efficient a method of signal transmissionis, the more nearly it is able to use the full Shannon capacity of thechannel. A channel with multiple transmit antennas and multiple receiveantennas which uses all possible signal paths between each pair oftransmit and receive antennas, referred to as a multiple input multipleoutput (“MIMO”) channel, is known to have a higher Shannon capacityunder certain channel conditions than a similar channel which uses onlyone transmit-receive antenna pair.

[0011] For signal reception, the signal must be demodulated and decoded.There are many methods of decoding convolutional codes, also referred toas “detection.” One method of decoding convolutional codes that uses thetrellis diagram representation is Viterbi decoding. In the trellisdiagram, each path through the trellis corresponds to a possible encodedsequence from the convolutional coder and the original informationsequence that generated the encoded sequence. The Viterbi algorithm usesthe encoded sequence actually received to determine a value of a metricfor some of the paths through the trellis and to eliminate other pathsfrom consideration. Finally, the decoder chooses a path through thetrellis with the most favorable value of the metric, and thecorresponding information sequence is thereby decoded. Thus, the Viterbidecoder provides maximum likelihood detection, as known in the art.

[0012] As stated above, one of the objects of coding is to protect theinformation in the user's signal from errors caused by variousphenomena, for example, multipath fading. Another collection oftechniques which can be used to increase signal reliability is referredto as “diversity.” Simply stated, diversity exploits the random natureof radio propagation by supplying to the receiver several replicas ofthe same information signal transmitted over independently fading—i.e.highly uncorrelated—signal paths for communication. For example, if oneradio signal path undergoes a deep fade, another independent path mayhave a strong signal. By having more than one path to select from thesignal to noise ratio of the information signal can be improved. Oneimplementation of diversity is the RAKE receiver, which employs severalantennas at the receiver to provide a selection of different signalpaths. A shortcoming of the RAKE receiver is that its effectivenessbreaks down at high data rates. One means of counteracting the effectsof time dispersion or multipath is the use of orthogonal frequencydivision multiplexing (“OFDM”) as known in the art. OFDM works well athigh data rates and thus avoids the shortcoming of ineffectiveness athigh data rates with the RAKE receiver.

[0013] A further collection of techniques which can be used to increasesignal reliability is referred to as “power control”. Simply stated,power control adjusts the power of the signal at the transmitter whilethe signal is being transmitted in order to compensate for varyingconditions in the communication channel, such as relative movement ofdifferent users and multipath fading. Power control relies on thetransmission of information regarding the condition of the channel, or“channel state information” (CSI) from the receiving unit back to thetransmitter. Thus, power control is a CSI technique. There are other CSItechniques which involve, for example, the use of separate “pilotsignals” and “training periods” of signal transmission. Diversitytechniques, on the other hand are non-CSI techniques, in that noseparate transmission of channel condition information is required fortheir implementation. In general, non-CSI techniques can be simpler andless costly to implement because non-CSI techniques avoid the complexityof transmitting channel state information.

[0014] Moreover, non-CSI techniques have an advantage over CSItechniques in that non-CSI techniques avoid incurring the “overhead” oftransmitting channel state information, i.e. non-user information, onthe channel. To the extent that channel capacity, i.e. the Shannoncapacity for a given SNR per bit, is used to transmit non-userinformation, i.e. CSI, less channel capacity is available fortransmitting user information, and the effective bandwidth efficiency ofthe transmission is, therefore, reduced. A channel which is not stableor for which channel conditions do not change slowly enough can requiretransmitting channel state information at high data rates to keep upwith changes in channel condition in order for the transmitter to beable to make effective use of the channel state information. Thus,non-CSI techniques can provide an advantage for mobile communicationswhere channel conditions are subject to rapid change.

[0015] The advantages of increased channel capacities of MIMO channelshave been used in conjunction with a number of CSI techniques. The useof non-CSI techniques such as coding, diversity, and OFDM can also beused to improve the error performance and “throughput”, i.e. the datarate of user information, for wireless communications. Thus, there is aneed in the art for taking advantage of the increased capacity of MIMOchannels by increasing the effective bandwidth efficiency oftransmission in MIMO channels while avoiding the disadvantages oftransmitting channel state information. There is also a need in the artto provide improvements in error performance, data rate, and capacity ofwireless communications in MIMO channels by exploiting increasedbandwidth efficiency.

SUMMARY

[0016] The present invention is directed to method and system forincreased bandwidth efficiency in multiple input—multiple outputchannels. The invention overcomes the need in the art for increasingbandwidth efficiency while avoiding the disadvantages of transmittingchannel state information. Also the invention provides improvements inerror performance, data rate, and capacity of wireless communications inmultiple input multiple output channels.

[0017] In one aspect of the invention an input bit stream is supplied toa trellis code block. For example, the trellis code block can performconvolutional coding using a rate 6/7 code. The output of the trelliscode block is then modulated using, for example, trellis codedquadrature amplitude modulation with 128 signal points or modulationsymbols. The sequence of modulation symbols thus generated can bediversity encoded. The diversity encoding can be either a space timeencoding, for example, or a space frequency encoding. The sequence ofmodulation symbols, or the sequence of diversity encoded modulationsymbols, is fed to two or more orthogonal Walsh covers. For example,replicas of the modulation symbol sequences can be provided to increasediversity, or demultiplexing the modulation symbol sequences can be usedto increase data transmission rate or “throughput”. The outputs of theWalsh covers are fed as separate inputs into a communication channel.The communication channel can be, for example, a multiple input multipleoutput channel in a WCDMA communication system.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 illustrates, in block diagram form, one example ofcommunication channel multiple input orthogonality for one embodiment ofthe present invention in an exemplary communication system.

[0019]FIG. 2 illustrates, in block diagram form, an example ofcommunication channel multiple input orthogonality for anotherembodiment of the present invention in an exemplary communicationsystem.

[0020]FIG. 3 illustrates in block diagram form, one example ofcommunication channel multiple input orthogonality with Alamoutitransmit diversity for one embodiment of the present invention in anexemplary communication system.

[0021]FIG. 4 illustrates, in block diagram form, an example ofcommunication channel multiple input orthogonality with Alamoutitransmit diversity for another embodiment of the present invention in anexemplary communication system.

[0022]FIG. 5 illustrates, in block diagram form, an example of receiverprocessing for use in conjunction with the examples of multiple inputorthogonality given in either of FIGS. 1 or 2.

[0023]FIG. 6 illustrates, in block diagram form, an example of receiverprocessing for use in conjunction with the examples of multiple inputorthogonality with Alamouti transmit diversity given in either of FIGS.3 or 4.

[0024]FIG. 7 illustrates, in block diagram form, an example ofcommunication channel multiple input orthogonality and diversity usingorthogonal frequency division multiplexing for one embodiment of thepresent invention in an exemplary communication system.

[0025] ICF FIG. 8 illustrates, in block diagram form, an example ofreceiver processing for use in conjunction with the example of multipleinput orthogonality and diversity using orthogonal frequency divisionmultiplexing given in FIG. 7.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] The presently disclosed embodiments are directed to method andsystem for increased bandwidth efficiency in multiple input—multipleoutput channels. The following description contains specific informationpertaining to the implementation of the presently disclosed embodiments.One skilled in the art will recognize that the presently disclosedembodiments may be implemented in a manner different from thatspecifically discussed in the present application. Moreover, some of thespecific details of the disclosed embodiments are not discussed in ordernot to obscure the invention. The specific details not described in thepresent application are within the knowledge of a person of ordinaryskill in the art.

[0027] The drawings in the present application and their accompanyingdetailed description are directed to merely example embodiments. Tomaintain brevity, other embodiments which use the principles of thepresent invention are not specifically described in the presentapplication and are not specifically illustrated by the presentdrawings.

[0028]FIG. 1 illustrates an example of orthogonality of multiple inputsto a communication channel in accordance with one embodiment. Exemplarysystem 100 shown in FIG. 1 constitutes part of a transmitter which maygenerally reside in a base station, gateway, or satellite repeater whencommunication is taking place in a forward channel. Exemplary system 100can be part of a base station transmitter, for example, in a widebandcode division multiple access (WCDMA) communication system. A WCDMAcommunication system is also referred to as a “spread spectrumcommunication system”.

[0029] In exemplary system 100 shown in FIG. 1, input bit stream 101contains the user's signal which includes information that is to betransmitted across the communication channel. The communication channelcan be, for example, radio frequency transmission between transmit andreceive antennas in a wireless communication system, including all thesignal paths between multiple transmit antennas and multiple receiveantennas. In this example, a transmit antenna is referred to as an inputto the communication channel and a receive antenna is referred to as anoutput of the communication channel. A communication system in whichthere is more than one input or more than one output to a communicationchannel is also referred to as a multiple input multiple output (“MIMO”)system.

[0030] Continuing with FIG. 1, input bit stream 101 is supplied to“trellis code” block 102. Trellis code block 102 performs convolutionalcoding, as described above, on input bit stream 101. In one embodimenttrellis code block 102 performs convolutional coding with a code rate ofthe form (n−1)/n, also referred to as a “rate (n−1)/n trellis code”. Asstated above, the code rate is equal to the input rate of informationbeing coded divided by the output rate of coded information, orequivalently the ratio of the number of bits of input to the number ofbits of output. For example, in one embodiment a rate 6/7 trellis codeis used, thus, there are 7 bits of output for each 6 bits of input totrellis code block 102.

[0031] Trellis code block 102 works in conjunction with QAM (quadratureamplitude modulation) block 104. The combined effect of trellis codeblock 102 and QAM block 104 is to convert input bit stream 101 into asequence of modulation symbols, also referred to as a “modulation symbolsequence”. Modulation symbols can be represented as signal points in acomplex phase space as described in an article titled “Channel Codingwith Multilevel/Phase Signals” by G. Ungerboeck, IEEE Transactions inInformation Theory, vol. IT-28, pp 55-67, January 1982. In oneembodiment, a trellis coded quadrature amplitude modulation with 128modulation symbols, or signal points, is used. A trellis codedquadrature amplitude modulation with 128 modulation symbols is alsoreferred to as 128-QAM. Other quadrature amplitude modulations can beused, for example, 8-QAM and 32-QAM. Moreover, other types ofmultilevel/phase modulations can be used, such as pulse amplitudemodulation (“PAM”), phase shift keying (“PSK”), or differential phaseshift keying (“DPSK”). By using different modulation schemes, that is,varying the modulation scheme during operation of the system, differentdata rates or throughputs can be supported at a tradeoff in reliability.For example, 8-QAM can be used to increase the error reliability ofsignal transmission at a given signal transmit power but at a lower datatransmission rate in comparison to 128-QAM or 32-QAM, and conversely,32-QAM can be used to increase throughput or data rate at the cost ofdecreased reliability in error performance for the same signal powercompared to 8-QAM. The choice of modulation can be used in conjunctionwith choice of Walsh functions, discussed below, to further enhance thedifferent combinations of throughput and reliability which can beachieved.

[0032] As shown in FIG. 1, the modulation symbol sequence from QAM block104 is fed to each of Walsh covers 110, 112, 114, and 116. Walsh cover110 is labeled “Walsh 1”; Walsh cover 112 is labeled “Walsh 2”; Walshcover 114 is labeled “Walsh 3”; and Walsh cover 116is labeled “Walsh 4”.Each of Walsh covers 110, 112, 114, and 116 is labeled differently toindicate that a distinct Walsh function is used by each Walsh cover toachieve orthogonality of the four outputs of Walsh covers 110, 112, 114,and 116.

[0033] By way of background, in WCDMA communications, each distinctsignal is spread to allow many signals to simultaneously occupy the samebandwidth without significantly interfering with one another. Thesignals can originate from different users, or as in the present exampleshown in FIG. 1, the signal can be replicated, for example, for thepurpose of achieving diversity, as explained above. One means ofspreading is the application of distinct “orthogonal” spreading codes orfunctions, such as Walsh functions, to each distinct signal.“Orthogonality” refers to lack of correlation between the spreadingfunctions. In a given spread spectrum communication system using Walshfunctions (also called Walsh code sequences), a pre-defined Walshfunction matrix having n rows of n chips each is established in advanceto define the different Walsh functions to be used to distinguishdifferent distinct signals. In the present example, each distinctreplica of the modulation symbol sequence is assigned a distinct Walshfunction. In other words, each distinct replica of the output signalfrom QAM block 104 is coded by a distinct Walsh code sequence in orderto separate each distinct signal from the others.

[0034] The general principles of CDMA communication systems, and inparticular the general principles for generation of spread spectrumsignals for transmission over a communication channel is described inU.S. Pat. No. 4,901,307 entitled “Spread Spectrum Multiple AccessCommunication System Using Satellite or Terrestrial Repeaters” andassigned to the assignee of the present invention. The disclosure inthat patent, i.e. U.S. Pat. No. 4,901,307, is hereby fully incorporatedby reference into the present application. Moreover, U.S. Pat. No.5,103,459 entitled “System and Method for Generating Signal Waveforms ina CDMA Cellular Telephone System” and assigned to the assignee of thepresent invention, discloses principles related to PN spreading, Walshcovering, and techniques to generate CDMA spread spectrum communicationsignals. The disclosure in that patent, i.e. U.S. Pat. No. 5,103,459, isalso hereby fully incorporated by reference into the presentapplication. Further, the presently disclosed embodiment utilizes timemultiplexing of data and various principles related to “high data rate”communication systems, and the presently disclosed embodiments can beused in “high data rate” communication systems, such as that disclosedin U.S. patent application Ser. No. 08/963,386 entitled “Method andApparatus for High Rate Packet Data Transmission” filed on Nov. 3, 1997,and assigned to the assignee of the present invention. The disclosure inthat patent application is also hereby fully incorporated by referenceinto the present application.

[0035] A Walsh function having n rows of n chips each is also referredto as a Walsh function matrix of order n. An example of a Walsh functionmatrix where n is equal to 4, i.e. a Walsh function matrix of order 4,is shown below: $\begin{pmatrix}1 & 1 & 1 & 1 \\1 & 0 & 1 & 0 \\1 & 1 & 0 & 0 \\1 & 0 & 0 & 1\end{pmatrix}$

[0036] In this example, there are 4 Walsh functions, each functionhaving 4 chips. Each Walsh function is one row in the above Walshfunction matrix. For example, the second row of the Walsh functionmatrix is the Walsh function having the sequence 1, 0, 1, 0. It is seenthat each Walsh function, i.e. each row in the above matrix, has zerocorrelation with any other row in the matrix. Stated differently,exactly half of the chips in every Walsh function differ from those inevery other Walsh function in the matrix.

[0037] Application of distinct orthogonal spreading functions, such asWalsh functions, to each distinct signal results in a transformation ofeach modulation symbol in each distinct signal into a respective spreadsequence of output chips, where each spread sequence of output chips isorthogonal with every other spread sequence of output chips. Using Walshfunctions, the transformation can be performed by XOR'ing eachmodulation symbol in each distinct signal with a sequence of chips in aparticular Walsh function. Using the second Walsh function in the aboveexample, i.e. the second row of the matrix, and XOR'ing a modulationsymbol of “a” with the second row of the matrix results in the spreadsequence of output chips: {overscore (a)}, a, {overscore (a)}, a, where“{overscore (a)}” denotes the binary complement of a. Thus, in thisillustrative example, each modulation symbol is spread into a spreadsequence of output chips having a length of 4. The number of outputchips produced for each input modulation symbol is called the spreadingfactor; in this illustrative example, the spreading factor is 4. Inpractice, Walsh functions of length 2 to 512 (i.e. Walsh functionshaving from 2 to 512chips in each Walsh code sequence) are used. Thus,spreading factors may range from 2 to 512 in practice.

[0038] Output 120 of Walsh cover 110, output 122 of Walsh cover 112,output 124 of Walsh cover 114, and output 126 of Walsh cover 116 are,thus, spread sequences of output chips which are mutually orthogonal.Each of outputs 120, 122, 124, and 126 is input separately to thecommunication channel. For example, each spread sequence of output chipscan be passed on to an FIR (“finite duration impulse response”) filterused for pulse shaping signals prior to their transmission using aseparate antenna for each signal input into the communication channel. Areceiver or receivers at the output of the communication channel, whichmay use a single antenna or multiple antennas, receives the signals. Forexample, the received signals can be passed through a receive FIR filterand then to a Walsh de-cover. The Walsh de-cover de-spreads the distinctspread sequences of output chips—i.e., undoes the Walsh functionspreading—by applying the distinct inverse functions of the originaldistinct Walsh function spreadings. Returning to the example used above(which has a spreading factor of 4), the second row of the matrix, 1, 0,1, 0, is again XOR'ed to the sequence of output chips, {overscore (a)},a, {overscore (a)}, a, to produce the data symbol sequence a, a, a, a,and thus the de-spread data symbol is “a”, the original input datasymbol. Thus, the original modulation symbol sequence can be recovered.

[0039] The modulation symbol sequence is input to a maximum likelihooddecoder, which can be, for example, a Viterbi decoder. Due toimperfections in the communication channel, the received modulationsymbol sequence may not be exactly identical to the original modulationsymbol sequence that was input to the communication channel. Simplystated, maximum likelihood detection determines a valid modulationsymbol sequence that is most likely to have produced the modulationsymbol sequence that is received by the maximum likelihood decoder.Thus, given a modulation symbol sequence, the maximum likelihood decoderdetermines a “best estimate” of the original modulation symbol sequenceand decodes the best estimate into an output information sequence.Because the best estimate is used, the output information sequencecontains a minimal number of errors in comparison to the originalinformation transmitted.

[0040] Thus, FIG. 1 shows one example of a system that can be used toprovide greater reliability of communications through use of orthogonaltransmit diversity in a multiple input multiple output communicationchannel.

[0041]FIG. 2 illustrates an example of orthogonality of multiple inputsto a communication channel in accordance with another embodiment.Exemplary system 200 shown in FIG. 2 constitutes part of a transmitterwhich may generally reside in a base station, gateway, or satelliterepeater when communication is taking place in a forward channel.Exemplary system 200 can be part of a base station transmitter, forexample, in a WCDMA communication system or spread spectrumcommunication system.

[0042] In exemplary system 200 shown in FIG. 2, input bit stream 201contains the signal of one or more users. The signal includesinformation that is to be transmitted across the communication channel.The communication channel can be, for example, radio frequencytransmission between transmit and receive antennas in a wirelesscommunication system comprising a multiple input multiple output, orMIMO, channel as described above.

[0043] Continuing with FIG. 2, input bit stream 201 is supplied to“trellis code” block 202. Trellis code block 202 performs convolutionalcoding, as described above, on input bit stream 201. In one embodimentdescribed here, trellis code block 202 performs rate (n−1)/n trelliscoding on input bit stream 201. For example, in one embodiment a rate6/7 trellis code is used.

[0044] Trellis code block 202 works in conjunction with QAM block 204.The combined effect of trellis code block 202 and QAM block 204 is toconvert input bit stream 201 into a modulation symbol sequence, in whichmodulation symbols can be represented as signal points in a complexphase space as stated above. In one embodiment, a trellis codedquadrature amplitude modulation with 128 modulation symbols, or 128-QAM,is used. As discussed above, other modulations can be used, and inconjunction with different Walsh functions, multiple combinations ofthroughput and reliability can be supported.

[0045] As shown in FIG. 2, a portion of the modulation symbol sequencefrom QAM block 204 is fed to each of Walsh covers 210, 212, 214, and 216in turn, by time demultiplexing the modulation symbol sequence. That is,a separate modulation symbol sequence is simultaneously fed to each ofWalsh covers 210, 212, 214, and 216. Because the capacity of thecommunication channel is still the same as for the example of FIG. 1,each separate Walsh cover can still input spread sequences to thecommunication channel at the same rate. To compensate for the increasedamount of information being fed to the communication channel, the outputrate of trellis code block 202 and QAM block 204 is increased, or“speeds up”, by a factor of four to feed each separate Walsh cover itsdistinct modulation symbol sequence. Thus, the maximum informationthroughput rate in the example of FIG. 2 is increased by a factor offour over that in the example of FIG. 1. There is a corresponding loss,however, of diversity in the example of FIG. 2 compared to the exampleof FIG. 1 where four replicas of the modulation symbol sequence aresimultaneously transmitted over the communication channel. In otherwords, some of the increased reliability in the example of FIG. 1 istraded for increased information throughput, or data rate, in theexample of FIG. 2.

[0046] Walsh cover 210 is labeled “Walsh 1”; Walsh cover 212 is labeled“Walsh 2”; Walsh cover 214 is labeled “Walsh 3”; and Walsh cover 216islabeled “Walsh 4”. Each of Walsh covers 210, 212, 214, and 216 islabeled differently to indicate that a distinct Walsh function is usedby each Walsh cover to achieve orthogonality of the four outputs ofWalsh covers 210, 212, 214, and 216. Output 220 of Walsh cover 210,output 222 of Walsh cover 212, output 224 of Walsh cover 214, and output226 of Walsh cover 216 are, thus, spread sequences of output chips whichare mutually orthogonal. Each of outputs 220, 222, 224, and 226 is inputseparately to the communication channel. For example, each spreadsequence of output chips can be passed on to an FIR filter used forpulse shaping signals prior to their transmission using a separateantenna for each signal input into the communication channel. A receiveror receivers at the output of the communication channel, which may use asingle antenna or multiple antennas, receives the signals, passes thesignals through receive FIR filters, Walsh de-covers the signals, anddecodes the modulation symbol sequence using maximum likelihood decodingas described above.

[0047] Thus, FIG. 2 shows an example of a system that can be used toprovide greater reliability of communications and increased data rate,with a trade off between increased reliability and increased data rate,through use of orthogonal transmit diversity in a multiple inputmultiple output communication channel.

[0048]FIG. 3 illustrates an example of orthogonality with Alamoutitransmit diversity of multiple inputs to a communication channel inaccordance with one embodiment. Exemplary system 300 shown in FIG. 3constitutes part of a transmitter which may generally reside in a basestation, gateway, or satellite repeater when communication is takingplace in a forward channel. Exemplary system 300 can be part of a basestation transmitter, for example, in a WCDMA communication system orspread spectrum communication system.

[0049] In exemplary system 300 shown in FIG. 3, input bit stream 301contains the user's signal which includes information that is to betransmitted across the communication channel. The communication channelcan be, for example, radio frequency transmission between transmit andreceive antennas in a wireless communication system comprising amultiple input multiple output, or MIMO, channel as described above.

[0050] Continuing with FIG. 3, input bit stream 301 is supplied to“trellis code” block 302. Trellis code block 302 performs convolutionalcoding, as described above, on input bit stream 301. In one embodimentdescribed here, trellis code block 302 performs rate (n−1)/n trelliscoding on input bit stream 301. For example, in one embodiment a rate6/7 trellis code is used.

[0051] Trellis code block 302 works in conjunction with QAM block 304.The combined effect of trellis code block 302 and QAM block 304 is toconvert input bit stream 301 into a modulation symbol sequence, in whichmodulation symbols can be represented as signal points in a complexphase space as stated above. In one embodiment, a trellis codedquadrature amplitude modulation with 128 modulation symbols, or 128-QAM,is used. As discussed above, other modulations can be used, and inconjunction with different Walsh functions, multiple combinations ofthroughput and reliability can be supported.

[0052] As shown in FIG. 3, the modulation symbol sequence from QAM block304 is provided to each of Alamouti blocks 306 and 308. The input andoutput of each of Alamouti blocks 306 and 308 is identical. As indicatedin FIG. 3, the modulation symbol sequence from QAM block 304 isalternately passed to the “A” inputs of Alamouti blocks 306 and 308 andthen to the “B” inputs of Alamouti blocks 306 and 308. Thus, themodulation symbol sequence from QAM block 304 is effectively groupedinto input groups of symbols, each input group containing 2 symbols,referred to in the present example as an “A symbol” and a “B symbol”.Each of Alamouti blocks 306 and 308 has a first and a second output. Foreach input group of modulation symbols, the first output of each ofAlamouti blocks 306 and 308 is the A symbol followed in sequence by thecomplex conjugate of the B symbol, indicated in FIG. 3 by the notation“(A, B*)”. Recall that each modulation symbol is represented as a signalpoint in complex phase space, i.e. as a complex number. For each inputgroup of modulation symbols, the second output of each of Alamoutiblocks 306 and 308 is the B symbol followed in sequence by the negativecomplex conjugate of the A symbol, indicated in FIG. 3 by the notation“(B, −A*)”. The theoretical justification and advantages of this schemeare described in an article titled “A Simple Transmit DiversityTechnique for Wireless Communications” by S. M. Alamouti, IEEE Journalon Select Areas in Communications, vol. 16, no. 8, pp 1451-58, October1998. The scheme of diversity encoding described in the present exampleis referred to as space time encoding. As described in the articlereferenced here, space frequency encoding can also be used in thepresent example, as is apparent to a person of ordinary skill in theart. The first output of Alamouti block 306 is fed to Walsh cover 310.The second output of Alamouti block 306 is fed to Walsh cover 312. Thefirst output of Alamouti block 308 is fed to Walsh cover 314. The secondoutput of Alamouti block 308 is fed to Walsh cover 316.

[0053] Walsh cover 310 is labeled “Walsh 1”; Walsh cover 312 is alsolabeled “Walsh 1”; Walsh cover 314 is labeled “Walsh 2”; and Walsh cover316is also labeled “Walsh 2”. Each of Walsh covers 310 and 312, islabeled identically, “Walsh 1”, to indicate that the same Walsh functionis used to spread both outputs of Alamouti block 306. Similarly, each ofWalsh covers 314 and 316, is labeled identically, “Walsh 2”, to indicatethat the same Walsh function is used to spread both outputs of Alamoutiblock 308. In other words, both Walsh covers in each pair of Walshcovers use the same Walsh function. Walsh covers 310 and 312 are labeleddifferently from Walsh covers 314 and 316 to indicate that distinctWalsh functions are used by each pair of Walsh covers to achievepairwise mutual orthogonality of the four outputs of Walsh covers 310,312, 314, and 316. In other words, pairwise mutual orthogonality refersto the condition that each pair of Walsh covers is orthogonal with everyother pair of Walsh covers. Output 320 of Walsh cover 310, output 322 ofWalsh cover 312, output 324 of Walsh cover 314, and output 326 of Walshcover 316 are, thus, spread sequences of output chips for which the pairof outputs 320 and 322 are orthogonal to the pair of outputs 324 and326. Each of outputs 320, 322, 324, and 326 is input separately to thecommunication channel. For example, each spread sequence of output chipscan be passed on to an FIR filter used for pulse shaping signals priorto their transmission using a separate antenna for each signal inputinto the communication channel. A receiver or receivers at the output ofthe communication channel, which may use a single antenna or multipleantennas, receives the signals, passes the signals through receive FIRfilters, Walsh de-covers the signals, and decodes the modulation symbolsequence using maximum likelihood decoding as described above.

[0054] The maximum information throughput rate in the example of FIG. 3is increased by a factor of two over that in the example of FIG. 1.Moreover, due to the use of Alamouti diversity encoding in the exampleof FIG. 3, there is an improvement in diversity compared to the exampleof FIG. 2. Thus, FIG. 3 shows an example of a system that can be used toprovide greater reliability of communications and increased data rate,with a trade off between increased reliability and increased data rate,through use of orthogonal transmit diversity in a multiple inputmultiple output communication channel.

[0055]FIG. 4 illustrates an example of orthogonality with Alamoutitransmit diversity of multiple inputs to a communication channel inaccordance with another embodiment. Exemplary system 400 shown in FIG. 4constitutes part of a transmitter which may generally reside in a basestation, gateway, or satellite repeater when communication is takingplace in a forward channel. Exemplary system 400 can be part of a basestation transmitter, for example, in a WCDMA communication system orspread spectrum communication system.

[0056] In exemplary system 400 shown in FIG. 4, input bit stream 401contains the signal of one or more users which includes information thatis to be transmitted across the communication channel. The communicationchannel can be, for example, radio frequency transmission betweentransmit and receive antennas in a wireless communication systemcomprising a multiple input multiple output, or MIMO, channel asdescribed above.

[0057] Continuing with FIG. 4, input bit stream 401 is supplied to“trellis code” block 402. Trellis code block 402 performs convolutionalcoding, as described above, on input bit stream 401. In one embodimentdescribed here, trellis code block 402 performs rate (n−1)/n trelliscoding on input bit stream 401. For example, in one embodiment a rate6/7 trellis code is used.

[0058] Trellis code block 402 works in conjunction with QAM block 404.The combined effect of trellis code block 402 and QAM block 404 is toconvert input bit stream 401 into a modulation symbol sequence, in whichmodulation symbols can be represented as signal points in a complexphase space as stated above. In one embodiment, a trellis codedquadrature amplitude modulation with 128 modulation symbols, or 128-QAM,is used. As discussed above, other modulations can be used, and inconjunction with different Walsh functions, multiple combinations ofthroughput and reliability can be supported.

[0059] As shown in FIG. 4, a portion of the modulation symbol sequencefrom QAM block 404 is fed to each of Alamouti blocks 406 and 408 inturn, by time demultiplexing the modulation symbol sequence. That is, aseparate modulation symbol sequence is simultaneously fed to each inputof each of Alamouti blocks 406 and 408. As indicated in FIG. 4, themodulation symbol sequence from QAM block 404 is alternately passed tothe “A” and “B” inputs of Alamouti block 406 and then to the “C” and “D”inputs of Alamouti block 408. Thus, the modulation symbol sequence fromQAM block 404 is effectively grouped into input groups of symbols, eachinput group containing 4 symbols, referred to in the present example asan “A symbol”, a “B symbol”, a “C symbol”, and a “D symbol”.

[0060] Each of Alamouti blocks 406 and 408 has a first and a secondoutput. For each input group of modulation symbols, the first output ofAlamouti block 406 is the A symbol followed in sequence by the complexconjugate of the B symbol, indicated in FIG. 4 by the notation “(A,B*)”. Recall that each modulation symbol is represented as a signalpoint in complex phase space, i.e. as a complex number. For each inputgroup of modulation symbols, the second output of Alamouti block 406 isthe B symbol followed in sequence by the negative complex conjugate ofthe A symbol, indicated in FIG. 4 by the notation “(B, −A*)”. For eachinput group of modulation symbols, the first output of Alamouti block408 is the C symbol followed in sequence by the complex conjugate of theD symbol, indicated in FIG. 4 by the notation “(C, D*)”. For each inputgroup of modulation symbols, the second output of Alamouti block 408 isthe D symbol followed in sequence by the negative complex conjugate ofthe C symbol, indicated in FIG. 4 by the notation “(D, −C*)”. The schemeof diversity encoding described in the present example is thus a variantof the space time encoding described in the above-referenced article byS. M. Alamouti. As described in the above-referenced article, spacefrequency encoding can also be used in the present example, as isapparent to a person of ordinary skill in the art. The first output ofAlamouti block 406 is fed to Walsh cover 410. The second output ofAlamouti block 406 is fed to Walsh cover 412. The first output ofAlamouti block 408 is fed to Walsh cover 414. The second output ofAlamouti block 408 is fed to Walsh cover 416.

[0061] Walsh cover 410 is labeled “Walsh 1”; Walsh cover 412 is alsolabeled “Walsh 1”; Walsh cover 414 is labeled “Walsh 2”; and Walsh cover416is also labeled “Walsh 2”. Each of Walsh covers 410 and 412, islabeled identically, “Walsh 1”, to indicate that the same Walsh functionis used to spread both outputs of Alamouti block 406. Similarly, each ofWalsh covers 414 and 416, is labeled identically, “Walsh 2”, to indicatethat the same Walsh function is used to spread both outputs of Alamoutiblock 408. Walsh covers 410 and 412 are labeled differently from Walshcovers 414 and 416 to indicate that distinct Walsh functions are used byeach pair of Walsh covers to achieve pairwise orthogonality of the fouroutputs of Walsh covers 410, 412, 414, and 416. Output 420 of Walshcover 410, output 422 of Walsh cover 412, output 424 of Walsh cover 414,and output 426 of Walsh cover 416 are, thus, spread sequences of outputchips for which the pair of outputs 420 and 422 are orthogonal to thepair of outputs 424 and 426. Each of outputs 420, 422, 424, and 426 isinput separately to the communication channel. For example, each spreadsequence of output chips can be passed on to an FIR filter used forpulse shaping signals prior to their transmission using a separateantenna for each signal input into the communication channel. A receiveror receivers at the output of the communication channel, which may use asingle antenna or multiple antennas, receives the signals, passes thesignals through receive FIR filters, Walsh de-covers the signals, anddecodes the modulation symbol sequence using maximum likelihood decodingas described above.

[0062] The maximum information throughput rate in the example of FIG. 4is increased by a factor of two over that in the example of FIG. 3.Thus, the information throughput rate in the example of FIG. 4 is theequal of the information throughput rate in the example of FIG. 2.Moreover, due to the use of Alamouti diversity encoding in the exampleof FIG. 4, there is an improvement in diversity compared to the exampleof FIG. 2. Thus, FIG. 4 shows an example of a system that can be used toprovide greater reliability of communications and increased data rate,with a trade off between increased reliability and increased data rate,through use of orthogonal transmit diversity in a multiple inputmultiple output communication channel.

[0063]FIG. 5 illustrates an example of receiver processing in accordancewith one embodiment. Exemplary system 500 shown in FIG. 5 constitutespart of a receiver which may generally reside in a subscriber unit ormobile unit when communication is taking place in a forward channel.Exemplary system 500 can be part of a subscriber unit receiver, forexample, in a WCDMA communication system or spread spectrumcommunication system.

[0064] In exemplary system 500 shown in FIG. 5, input signal 501 isreceived from a communication channel with 4 inputs. The communicationchannel can be, for example, radio frequency transmission betweenmultiple transmit and receive antennas in a wireless communicationsystem comprising a multiple input multiple output, or MIMO, channel asdescribed above. For a channel with 4 receive antennas, for example, a“copy” of exemplary system 500 would be present for each separatereceive antenna.

[0065] Continuing with FIG. 5, the receive processing shown in FIG. 5 isintended to be used with either of the transmit processes used in FIG. 1or FIG. 2. Thus, input signal 501 comprises a composite signal which hasbeen Walsh covered by each of Walsh 1, Walsh 2, Walsh 3, and Walsh 4 ofeither FIG. 1 or FIG. 2. Input signal 501 is passed to each of Walshdecovers 512, 514, 516, and 518.

[0066] Walsh decover 512 is labeled “Walsh 1”; Walsh decover 514 islabeled “Walsh 2”; Walsh decover 516 is labeled “Walsh 3”; and Walshdecover 518 labeled “Walsh 4”. Each of Walsh decovers 512, 514, 516, and518 is labeled so as to indicate that the same distinct Walsh functionis used by each Walsh decover to decover the corresponding transmitsignal from FIG. 1 or FIG. 2. Thus, the four outputs of Walsh decovers512, 514, 516, and 518 correspond to 4 replicas of the single modulationsymbol sequence that was transmitted in the case of FIG. 1, oralternatively, to 4 distinct modulation symbol sequences that weretransmitted in the case of FIG. 2. The replica or distinct modulationsymbol sequence output by Walsh decover 512 is passed to metricgeneration block 522, the output of Walsh decover 514 is passed tometric generation block 524, and so forth as indicated in FIG. 5.

[0067] Each of metric generation blocks 522, 524, 526, and 528 performsmetric generation, also referred to as “path metric” generation or“branch metric” generation, for input to a trellis decoder, which can bea Viterbi decoder, for example. For example, each of metric generationblocks 522, 524, 526, and 528 can multiply each output of the Walshdecover operation by the complex conjugate of each possible transmittedmodulation symbol, then multiply by the complex conjugate of theestimated complex path gain, take twice the real part of the result, andsubtract a bias term, comprising the square of the magnitude of the pathgain of the channel times the square of the magnitude of the possibletransmitted modulation symbol, to produce a metric value for input tothe trellis decoder. As shown in FIG. 5, the metric values output bymetric generation blocks 522, 524, 526, and 528 are all fed into trellisdecode block 532. Output 533 of trellis decode block 532 is a maximumlikelihood estimate, as discussed above, of the original input bitstream of one or more users, that is, input bit stream 101 in the caseof FIG. 1 or input bit stream 201 in the case of FIG. 2. Thus, FIG. 5shows an example of receiver processing for utilization of signalorthogonality in a multiple input multiple output communication channel.

[0068]FIG. 6 illustrates an example of receiver processing in accordancewith one embodiment. Exemplary system 600 shown in FIG. 6 constitutespart of a receiver which may generally reside in a subscriber unit ormobile unit when communication is taking place in a forward channel.Exemplary system 600 can be part of a subscriber unit receiver, forexample, in a WCDMA communication system or spread spectrumcommunication system.

[0069] In exemplary system 600 shown in FIG. 6, input signal 601 isreceived from a communication channel with 4 inputs. The communicationchannel can be, for example, radio frequency transmission betweenmultiple transmit and receive antennas in a wireless communicationsystem comprising a multiple input multiple output, or MIMO, channel asdescribed above. For a channel with 4 receive antennas, for example, a“copy” of exemplary system 600 would be present for each separatereceive antenna.

[0070] Continuing with FIG. 6, the receive processing shown in FIG. 6 isintended to be used with either of the transmit processes used in FIG. 3or FIG. 4. Thus, input signal 601 comprises a composite signal which hasbeen Walsh covered by each of Walsh 1, and Walsh 2 of either FIG. 3 orFIG. 4. Input signal 601 is passed to each of Walsh decovers 602 and604.

[0071] Walsh decover 602 is labeled “Walsh 1”, and Walsh decover 604 islabeled “Walsh 2”. Each of Walsh decovers 602 and 604 is labeled so asto indicate that the same distinct Walsh function is used by each Walshdecover to decover the corresponding transmit signal from FIG. 3 or FIG.4. Thus, the two outputs of Walsh decovers 602 and 604 correspond to areceived symbol pair from which each symbol of the received symbol paircan be estimated using the Alamouti technique referenced above.

[0072] By way of brief illustration, the symbol pair (A, B*) shown inFIG. 3 can be transmitted with a channel path gain of h₁ so that thepair “becomes” (h₁A, h₁B*) and the symbol pair (B, −A*) shown in FIG. 3can be transmitted with a channel path gain of h₂ so that the pair“becomes” (h₂B, −h₂A*). During the first time interval a certain amountof noise n(1) from the channel gets added to the received signal in thefirst time interval, referred to as r(1). The received signal in thefirst time interval is then:

r(1)=h ₁ A+h ₂ B+n(1).

[0073] During the second time interval a certain amount of noise n(2)from the channel gets added to the received signal in the second timeinterval, referred to as r(2). The received signal in the second timeinterval is then:

r(2)=h ₁ B* −h ₂ A*+n(2).

[0074] With the aid of signal delay elements, for example, to make thevalues of r(l) and r(2) available at the same point in time, the blocksmarked “first symbol Alamouti estimate” perform the following algebraicoperations on r(1) and r(2).

[0075] First symbol estimate=h₁*r(1)−h₂r*(2).

[0076] Similarly, the blocks marked “second symbol Alamouti estimate”perform the following algebraic operations on r(1) and r(2).

[0077] Second symbol estimate h₂*r(1)+h₁r*(2).

[0078] It can be shown using the algebra of complex numbers that:

[0079] First symbol estimate=(|h₁|²+|h₂|²)A+h₁*n(1)+h₂n(2), and

[0080] Second symbol estimate=(|h₁|²+|h₂|²)B+h₂*n(1)+h₁n(2).

[0081] Thus, “first symbol estimate” is an estimate of the symbol “A”and “second symbol estimate” is an estimate of the symbol “B”. Eachestimate contains a bias, (|h₁|²+|h₂|²), which can be offset by metricgeneration blocks 622, 624, 626, and 628.

[0082] In the case where the transmit process of FIG. 3 is used, bothWalsh 1 and Walsh 2 cover the symbol pair A, B. Thus, output symbol S₁of first symbol Alamouti estimate block 612 shown in FIG. 6 is anestimate of symbol “A”; output symbol S₂ of second symbol Alamoutiestimate block 614is an estimate of symbol “B”; output symbol S₃ is anestimate of symbol “A”; and output symbol S₄ is an estimate of symbol“B”, in the case of FIG. 3.

[0083] In the case where the transmit process of FIG. 4 is used, Walsh 1covers the symbol pair A, B and Walsh 2 covers the symbol pair C, D.Thus, output symbol S₁ of first symbol Alamouti estimate block 612 shownin FIG. 6 is an estimate of symbol “A”; output symbol S₂ of secondsymbol Alamouti estimate block 614 is an estimate of symbol “B”; outputsymbol S₃ of first symbol Alamouti estimate block 616 is an estimate ofsymbol “C”; and output symbol S₄ of second symbol Alamouti estimateblock 618 is an estimate of symbol “D”, in the case of FIG. 4.

[0084] The symbol estimates are passed to each of metric generationblocks 622, 624, 626, and 628 respectively as indicated in FIG. 6. Eachof metric generation blocks 622, 624, 626, and 628 performs metricgeneration, also referred to as “path metric” generation or “branchmetric” generation, for input to a trellis decoder, which can be aViterbi decoder, for example. For example, each of metric generationblocks 622, 624, 626, and 628 can multiply each modulation symbolestimate in the sequence of symbol estimates generated by each of theAlamouti estimate blocks by the complex conjugate of each possibletransmitted modulation symbol, take twice the real part of the result,and subtract a bias term, comprising (|h₁|²+|h₂|²) times the square ofthe magnitude of the possible transmitted modulation symbol, to producea metric value for input to the trellis decoder. As shown in FIG. 6, themetric values output by metric generation blocks 622, 624, 626, and 628are all fed into trellis decode block 632. Output 633 of trellis decodeblock 632 is a maximum likelihood estimate, as discussed above, ofeither the original user input bit stream in the case of FIG. 3 ormultiple users' input bit streams in the case of FIG. 4. Thus, FIG. 6shows an example of receiver processing for utilization of signalorthogonality with Alamouti transmit diversity in a multiple inputmultiple output communication channel.

[0085]FIG. 7 shows an example of how OFDM can be used in one embodiment.Exemplary system 700 shown in FIG. 7 constitutes part of a transmitterwhich may generally reside in a base station, gateway, or satelliterepeater when communication is taking place in a forward channel.Exemplary system 700 can be part of a base station transmitter, forexample, in a WCDMA communication system or spread spectrumcommunication system.

[0086] In exemplary system 700 shown in FIG. 7, input bit stream 701contains the signal of one or more users. The signal includesinformation that is to be transmitted across the communication channel.The communication channel can be, for example, radio frequencytransmission between transmit and receive antennas in a wirelesscommunication system comprising a multiple input multiple output, orMIMO, channel as described above. As shown in FIG. 7, input bit stream701 is supplied to “trellis coded QAM” block 702. Trellis coded QAMblock 202 performs convolutional coding and quadrature amplitudemodulation, as described above in relation to FIGS. 1 and 2, on inputbit stream 701.

[0087] Continuing with FIG. 7, the modulation symbol sequence fromtrellis coded QAM block 202 is fed to “frequency coding” block 704.Frequency coding block 704 feeds the modulation symbol sequence to eachof Walsh/Alamouti blocks 712, 714, and 716 by time demultiplexing themodulation symbol sequence. That is, a portion of the modulation symbolsequence is simultaneously fed to each of Walsh/Alamouti blocks 712,714, and 716 at a separate distinct frequency for each Walsh/Alamoutiblock. Each distinct separate frequency is also referred to as a“frequency bin”. The example shown in FIG. 7 uses only 3 separatefrequencies or frequency bins for the sake of simplicity and brevity inthe present illustrative example. Thus, the example of FIG. 7 shows only3 Walsh/Alamouti blocks, one for each frequency bin. In practice anydesired number of frequency bins can be used based on practicalconsiderations. The number of Walsh/Alamouti blocks can be adjustedaccordingly, as is apparent to a person of ordinary skill in the art.

[0088] Moreover, the present example is based on straightforwarddemultiplexing of the modulation symbol sequence. That is, a symbol fromthe modulation symbol sequence is coded into the first frequency bin,the subsequent symbol is coded into the second frequency bin, the nextsubsequent symbol is coded into the third frequency bin, and the nextsubsequent symbol is coded once again into the first frequency bin, andso forth for each subsequent symbol. Many variations on this scheme arepossible, for example, a replica of the entire modulation symbolsequence can be coded into each frequency bin. Any of a number of othertechniques known in the art can also be used, such as symbol repetitionand symbol interleaving, for example. The details of implementing thesetechniques are apparent to a person of ordinary skill in the art, andare therefore not presented here.

[0089] Continuing with FIG. 7, each of Walsh/Alamouti blocks performsthe processing subsequent to trellis coded modulation as described abovein connection with any of FIGS. 1 through 4. For example, if theprocessing of FIG. 4 were chosen for each of Walsh/Alamouti blocks 712,714, and 716, then each of Walsh/Alamouti blocks 712, 714, and 716performs the processing indicated in FIG. 4 for Alamouti blocks 406 and408, and Walsh covers 410, 412, 414, and 416. For example, in the caseof Walsh/Alamouti block 712, the modulation symbol sequence that isinput to Walsh/Alamouti block 712 is first demultiplexed into twoAlamouti blocks corresponding to Alamouti block 406 and Alamouti block408 of FIG. 4. The output of Walsh/Alamouti block 712 is, thus, theoutput of 4 Walsh covers corresponding to Walsh covers 410, 412, 414,and 416. Thus, the output of Walsh/Alamouti block 712 is 4 pairwiseorthogonal spread sequences of output chips as described above inrelation to FIG. 4. Thus, the orthogonal transmit diversity technique,in any of the forms described above, is applied to the modulation symbolsequence for each frequency bin.

[0090] The four outputs of each of Walsh/Alamouti blocks 712, 714, and716 are fed to each of “inverse FFT and cyclic prefix” blocks 722, 724,726, and 728. That is, each of the four inverse FFT and cyclic prefixblocks 722, 724, 726, and 728 has 3 distinct spread sequences of outputchips, one for each frequency bin, as input. The present example, aswith the examples of FIGS. 1 through 4, assumes that four separateantennas are used as inputs to the multiple input multiple outputcommunication channel. It is manifest that any practical number ofinputs to the multiple input multiple output channel can be used. Forexample, a channel with 8 inputs would require 8 inverse FFT and cyclicprefix blocks, and each Walsh/Alamouti block would be required toprovide 8 outputs. The required changes to the examples given areapparent to a person of ordinary skill in the art. Thus, each inverseFFT and cyclic prefix block performs inverse FFT processing on theoutput of all of the frequency bins, i.e. the spread sequences of outputchips. The inverse FFT operation is performed once for each chip periodof the spread sequences of output chips. The cyclic prefix is atechnique, known in the art, which adds a certain amount of knownsamples to the inverse FFT to account for time dispersion in thechannel. The amount of cyclic prefix is determined based on the maximumtime dispersion among all signal paths in the multiple input multipleoutput channel. Thus, each of inverse FFT and cyclic prefix blocks 722,724, 726, and 728 transform the spread sequences of output chips fromall of the frequency bins from the frequency domain into the timedomain.

[0091] Output 723 of inverse FFT and cyclic prefix block 722, output 725of inverse FFT and cyclic prefix block 724, output 727 of inverse FFTand cyclic prefix block 726, output 729 of inverse FFT and cyclic prefixblock 728 are, thus, spread sequences of output chips in the timedomain. Each of outputs 723, 725, 727, and 729 is input separately tothe communication channel. For example, each spread sequence of outputchips can be passed on to an FIR filter used for pulse shaping signalsprior to their transmission using a separate antenna for each signalinput into the communication channel. A receiver or receivers at theoutput of the communication channel, which may use a single antenna ormultiple antennas, receives the signals, passes the signals throughreceive FIR filters, and performs other processing as described below inrelation to FIG. 8.

[0092] Thus, FIG. 7 shows an example of a system that can be used tocounteract the effects of time dispersion in a multiple input multipleoutput communication channel through use of OFDM combined withorthogonal transmit diversity. Thus, the system provides greaterreliability of communications and increased data rate for increasedbandwidth efficiency of signal transmission in a multiple input multipleoutput channel.

[0093]FIG. 8 illustrates an example of receiver processing whichincorporates OFDM in accordance with one embodiment Exemplary system 800shown in FIG. 8 constitutes part of a receiver which may generallyreside in a subscriber unit or mobile unit when communication is takingplace in a forward channel. Exemplary system 800 can be part of asubscriber unit receiver, for example, in a WCDMA communication systemor spread spectrum communication system.

[0094] In exemplary system 800 shown in FIG. 8, input signals 801, 803,805, and 807 are received from a multiple input multiple outputcommunication channel. The communication channel can be, for example,radio frequency transmission between multiple transmit and receiveantennas in a wireless communication system comprising a multiple inputmultiple output, or MIMO, channel as described above. For the exampleused in the present application, a channel with 4 transmit antennas and4 receive antennas is illustrated. Thus each of input signals 801, 803,805, and 807 is received on a separate receive antenna.

[0095] Continuing with FIG. 8, the receive processing shown in FIG. 8 isintended to be used with the transmit processes used in FIG. 7. Thetransmit process of FIG. 7 can incorporate any of the transmit processesdescribed in connection with FIG. 1, FIG. 2, FIG. 3, or FIG. 4, as notedabove. Thus, input signals 801, 803, 805, and 807 comprise time domainspread sequences of output chips from all of the frequency bins. Inputsignal 801 is passed to FFT block 802, input signal 803 is passed to FFTblock 804, input signal 805 is passed to FFT block 806, and input signal807 is passed to FFT block 808. Each of FFT blocks 802, 804, 806, and808 performs an FFT operation once for each chip period of the spreadsequences of output chips to transform the spread sequences of outputchips from the time domain into the frequency domain to fill all of thefrequency bins. The spread sequences of output chips in the frequencydomain are then passed to “Walsh/Alamouti and metric generation” blocks812, 814, and 816. There is one Walsh/Alamouti and metric generationblock for each frequency bin. Recall that the example used in thepresent application has 3 frequency bins, thus there are 3Walsh/Alamouti and metric generation blocks. Each block performsreceiver processing for generation of metric values as described abovein relation to FIGS. 5 and 6. The processing of FIG. 5 or FIG. 6,respectively, is used depending on whether the transmit processingscheme used is that of FIGS. 1 or 2 or that of FIGS. 3 or 4. The metricvalues output by each of Walsh/Alamouti and metric generation blocks812, 814, and 816 are passed to “frequency decode” block 822. Frequencydecode block 822 time multiplexes the metric values to undo the timedemultiplexing performed in the example used in the present application,which was discussed above in relation to FIG. 7. If other techniques,for example, interleaving, were used, then frequency decode block 822would include the corresponding deinterleaving, for example, as isapparent to a person of ordinary skill in the art. The frequency decodedmetric values are then passed to trellis decode block 832, which cancomprise a Viterbi decoder, for example, to decode the modulation symbolsequence using maximum likelihood decoding as described above. Thus,FIG. 8 shows an example of receiver processing for utilization of signalorthogonality with Alamouti transmit diversity combined with orthogonalfrequency division multiplexing in a multiple input multiple outputcommunication channel.

[0096] It is appreciated by the above description that the disclosedembodiments provide a method and system for increased bandwidthefficiency in multiple input multiple—output channels using multipleinput multiple output techniques for wireless communications in a WCDMAsystem. According to an embodiment described above, user's informationis transmitted between multiple transmit antennas and multiple receiveantennas while maintaining orthogonality between antennas as well asbetween users. Moreover, according to an embodiment described above,diversity is also achieved while maintaining orthogonality betweenantennas as well as between users. Although the disclosed embodimentsare described as applied to communications in a WCDMA system, it will bereadily apparent to a person of ordinary skill in the art how to applythe disclosed embodiments in similar situations where orthogonaltransmit diversity is needed for use with multiple transmit and receiveantennas or multiple communication inputs and outputs.

[0097] From the above description, it is manifest that varioustechniques can be used for implementing the concepts of the presentlydisclosed embodiments without departing from its scope. Moreover, whilethe presently disclosed embodiments have been described with specificreference to certain embodiments, a person of ordinary skill in the artwould recognize that changes can be made in form and detail withoutdeparting from the spirit and the scope of the presently disclosedembodiments. For example, the quadrature amplitude modulation QAMpresented in an embodiment described here can be replaced by other typesof modulation such as phase shift keying (“PSK”). Also, for example, thespace time diversity encoding, presented in one embodiment describedhere, can be replaced by space frequency diversity encoding. Thedescribed embodiments are to be considered in all respects asillustrative and not restrictive. It should also be understood that thepresently disclosed embodiments are not limited to the particularembodiments described herein, but is capable of many rearrangements,modifications, and substitutions without departing from the scope of theinvention.

[0098] Thus, method and system for increased bandwidth efficiency inmultiple input—multiple output channels have been described.

What is claimed is:
 1. A method comprising steps of: supplying an inputbit stream to a trellis code block; modulating an output of said trelliscode block so as to provide a modulation symbol sequence; feeding saidmodulation symbol sequence to a plurality of Walsh covers, wherein eachof said plurality of Walsh covers outputs one of a plurality of spreadsequences of output chips; transmitting said plurality of spreadsequences of output chips over a channel.
 2. The method of claim 1wherein said feeding step comprises feeding a replica of said modulationsymbol sequence to each of said plurality of Walsh covers.
 3. The methodof claim 1 wherein said feeding step comprises time demultiplexing saidmodulation symbol sequence so as to simultaneously provide a portion ofsaid modulation symbol sequence to each of said plurality of Walshcovers.
 4. The method of claim 1 wherein said plurality of Walsh coverscomprise mutually orthogonal Walsh covers.
 5. The method of claim 1wherein said modulating step comprises modulating said output of saidtrellis code block using trellis coded quadrature amplitude modulation.6. The method of claim 1 wherein said trellis code block performs rate(n−1)/n trellis coding on said input bit stream.
 7. The method of claim1 further comprising steps of: frequency coding said modulation symbolsequence after said modulating step and before said feeding step;performing inverse FFT and cyclic prefix processing after said feedingstep and before said transmitting step.
 8. The method of claim 1 whereinsaid channel comprises a multiple input multiple output channel.
 9. Amethod comprising steps of: supplying an input bit stream to a trelliscode block; modulating an output of said trellis code block so as toprovide a first modulation symbol sequence; diversity encoding saidfirst modulation symbol sequence so as to generate a second modulationsymbol sequence; feeding said second modulation symbol sequence to aplurality of Walsh covers, wherein each of said plurality of Walshcovers outputs one of a plurality of spread sequences of output chips;transmitting said plurality of spread sequences of output chips over achannel.
 10. The method of claim 9 wherein said diversity encoding isspace time encoding.
 11. The method of claim 9 wherein said diversityencoding is space frequency encoding.
 12. The method of claim 9 whereinsaid feeding step comprises feeding a replica of said second modulationsymbol sequence to a pair of said plurality of Walsh covers.
 13. Themethod of claim 9 wherein said diversity encoding step comprisesdemultiplexing said first modulation symbol sequence, whereby a portionof said first modulation symbol sequence corresponding to a pair of saidplurality of Walsh covers is simultaneously diversity encoded, so as toprovide a portion of said second modulation symbol sequence to each ofsaid plurality of Walsh covers.
 14. The method of claim 9 wherein saidplurality of Walsh covers comprise pairwise mutually orthogonal Walshcovers.
 15. The method of claim 9 wherein said modulating step comprisesmodulating said output of said trellis code block using trellis codedquadrature amplitude modulation.
 16. The method of claim 1 furthercomprising steps of: frequency coding said modulation symbol sequenceafter said modulating step and before said feeding step; performinginverse FFT and cyclic prefix processing after said feeding step andbefore said transmitting step.
 17. The method of claim 9 wherein saidchannel comprises a multiple input multiple output channel.
 18. A systemcomprising: a trellis code block configured to encode an input bitstream; a modulator configured to receive an output of said trellis codeblock to provide a modulation symbol sequence; a plurality of Walshcovers, wherein said modulation symbol sequence is fed to said pluralityof Walsh covers and each of said plurality of Walsh covers outputs oneof a plurality of spread sequences of output chips; said systemconfigured to transmit said plurality of spread sequences of outputchips over a channel.
 19. The system of claim 18 wherein a replica ofsaid modulation symbol sequence is fed to each of said plurality ofWalsh covers.
 20. The system of claim 18 wherein said modulation symbolsequence is time demultiplexed so as to simultaneously provide a portionof said modulation symbol sequence to each of said plurality of Walshcovers.
 21. The system of claim 18 wherein said plurality of Walshcovers comprise mutually orthogonal Walsh covers.
 22. The system ofclaim 18 wherein said modulator is configured to modulate said output ofsaid trellis code block using trellis coded quadrature amplitudemodulation.
 23. The system of claim 18 wherein said trellis code blockperforms rate (n−1)/n trellis coding on said input bit stream.
 24. Thesystem of claim 18 further comprising: a frequency coder configured tofrequency code said modulation symbol sequence so as to simultaneouslyprovide a portion of said modulation symbol sequence to each of saidplurality of Walsh covers; an inverse FFT processor configured totransform said output of each of said plurality of Walsh covers from thefrequency domain into the time domain so as to provide said plurality ofspread sequences of output chips.
 25. The system of claim 18 whereinsaid channel comprises a multiple input multiple output channel.
 26. Asystem comprising: a trellis code block configured to encode an inputbit stream; a modulator configured to receive an output of said trelliscode block to provide a first modulation symbol sequence; an Alamoutiblock configured to diversity encode said first modulation symbolsequence so as to generate a second modulation symbol sequence; aplurality of Walsh covers, wherein said second modulation symbolsequence is fed to said plurality of Walsh covers and each of saidplurality of Walsh covers outputs one of a plurality of spread sequencesof output chips; said system configured to transmit said plurality ofspread sequences of output chips over a channel.
 27. The system of claim26 wherein said Alamouti block is configured to diversity encode usingspace time encoding.
 28. The system of claim 26 wherein said Alamoutiblock is configured to diversity encode using space frequency encoding.29. The system of claim 26 wherein a replica of said second modulationsymbol sequence is fed to a pair of said plurality of Walsh covers. 30.The system of claim 26 wherein said first modulation symbol sequence isdemultiplexed, whereby a portion of said first modulation symbolsequence corresponding to a pair of said plurality of Walsh covers issimultaneously diversity encoded, so as to provide a portion of saidsecond modulation symbol sequence to each of said plurality of Walshcovers.
 31. The system of claim 26 wherein said plurality of Walshcovers comprises pairwise mutually orthogonal Walsh covers.
 32. Thesystem of claim 26 wherein said modulating step comprises modulatingsaid output of said trellis code block using trellis coded quadratureamplitude modulation.
 33. The system of claim 26 further comprising: afrequency coder configured to frequency code said modulation symbolsequence so as to simultaneously provide a portion of said modulationsymbol sequence to each of said plurality of Walsh covers; an inverseFFT processor configured to transform said output of each of saidplurality of Walsh covers from the frequency domain into the time domainso as to provide said plurality of spread sequences of output chips. 34.The system of claim 26 wherein said channel comprises a multiple inputmultiple output channel.